Self mode-locking semiconductor disk laser

ABSTRACT

The present invention describes a self mode locking laser and a method for self mode locking a laser. The laser ( 1 ) comprises a resonator terminated by first ( 3 ) and second ( 4 ) mirrors and folded by a third mirror ( 5 ). The third mirror comprises a single distributed Bragg reflector ( 17 ) upon which is mounted a multilayer semiconductor gain medium ( 18 ) and which includes at least one quantum well layer and an optical Kerr lensing layer ( 22 ). Self mode locking may be achieved by configuring the laser resonator such that the lensing effect of the Kerr lensing layer acts to reduce an astigmatism deliberately introduced to the cavity mode. The self mode locking of the laser may be further enhanced by selecting the length of the resonator such that a round trip time of a cavity mode is matched with an upper-state lifetime of one or more semiconductor carriers located within the gain medium.

The present invention relates to the field of semiconductor lasers andin particular to a semiconductor disc laser (SDL) configured to emitultra short pulses of radiation.

It is noted that SDLs are also known in the art as Vertical ExternalCavity Emitting Lasers (VECSELs) or Optically Pumped SemiconductorLasers (OPSLs). Therefore the term semiconductor disc laser (SDL) whenused throughout the present description is used to refer to each ofthese systems.

The term “ultra short” pulses as used within the following descriptionrefers to pulses having a duration from about 100 picoseconds (ps) downto a few femtoseconds (fs).

Ultra short pulses of optical radiation generated by laser sources areemployed in a range of scientific, instrumentation and nonlinear opticsapplications. One particular application for these ultra short pulses isin the field of nonlinear microscopy for example Two-Photon ExcitedFluorescence (TPEF) microscopy or other similar multi-photon microscopytechniques. Historically, Ti:sapphire laser sources have been employedto perform these nonlinear microscopy techniques due to the inherentlarge tuneable ranges (700 nm to 1,000 nm) and peak powers available tosuch gain mediums. A Ti:sapphire laser system is generally opticallypumped at wavelength in the green region of the spectrum and thereforethese systems are typically pumped with frequency-doubled solid statelasers having a neodymium-doped gain medium such as neodymium-doped YAG(Nd:YAG) or neodymium-doped yttrium orthovanadate (Nd:YVO₄) whereinradiation having a fundamental wavelength of about 1064 nm is convertedinto radiation having a wavelength of about 532 nm by frequency-doublingtechniques. As a result, the above described solid state ultrafastlasers are relatively complex, bulky, have high maintenance requirementsand most significantly are expensive to produce.

As an alternative to Ti:sapphire laser sources, it is known in the artto employ ultrafast semiconductor saturable absorber mirrors (SESAM) formode locking diode pumped solid state-lasers. Compact designs have beenrealised by “folding” long cavities or by increasing the repetition rateof the lasers which naturally allows for a shorter cavity length. Themain drawback of systems that employ SESAMs is that they are stillrelatively complex to produce and maintain.

More recently compact ultrafast chromium-doped laser systems such asCr:LiCAF, Cr:LiSAF, and Cr:LiSGAF lasers have been developed andemployed for nonlinear TPEF microscopy. Although average powers of up to500 mW have been demonstrated, systems based on such materials are oftenlimited in their ability to sufficiently scale their average power.

Other alternative sources based on fibre lasers and semiconductor laserdiode with amplification schemes have also been successfullydemonstrated as compact lasers for nonlinear microscopy applications.Fibre lasers can be employed to generate very short pulses via passivemode locking techniques. They are however limited to operating atwavelengths around 1030 nm and 1550 nm and the second harmonics producedfrom these fundamental wavelengths. Within semiconductor lasers withamplification schemes e.g. gain-switched sources based on verticalcavity surfaces emitting lasers (VECSELs); gain switched InGaAsPDistributed Feedback-Bragg (DFB), laser diode; and external cavitymode-locked laser diode consisting of multiple quantum wells (AlGaAs),the simplicity or compactness of the system is compromised as they allrequire several stages to compress and or amplify the generated opticalpulses.

One key aspect of optimising a compact laser system for nonlinearmicroscopy applications is the critical trade off between repetitionrate of the laser and the multi-photon signal strength generated. Thesignal strength in TPEF microscopy scales as the product of the peakpower times the average power (assuming image spot size, absorption,sample, detection path, etc. remain constant). An example of an ultrashort pulse mode-locked SDL is described in US patent publication numberUS 2009/0290606. This document describes optically pumping the SDL gainstructure with optical pulses, delivered at a pulse repetition frequencycorresponding to a resonant frequency of the laser resonator. Theresonator additionally includes a passive mode locking arrangement suchas an optical element that exhibits a strong optical Kerr effect, asecond harmonic generating nonlinear crystal which acts in conjunctionwith the output coupler, or an output coupler which comprises asemiconductor saturable absorber mirror (SESAM). Although the describedcombination of optically pumping the gain structure with an optical pumppulse source in conjunction with a passive mode locking scheme iscapable of generating pulses ranging from 100 ps to ˜100 fs, theincorporation of such elements adds to the overall complexity andexpense of the ultra short pulse source.

It is therefore an object of an embodiment of the present invention toobviate or at least mitigate the foregoing disadvantages of the ultrashort pulse sources of radiation known in the art.

It is a further object of an embodiment of the present invention toprovide a self-mode locking semiconductor disc laser system.

SUMMARY OF INVENTION

According to a first aspect of the present invention there is provided aself mode locking laser the laser comprising

a resonator terminated by first and second mirrors and folded by a thirdmirror, the third mirror surmounted by a multilayer semiconductor gainmedium including at least one quantum well layer and an optical Kerrlensing layer,wherein a length of the resonator is selected such that a round triptime of a cavity mode corresponds with an upper-state lifetime of one ormore semiconductor carriers located within the gain medium.

The above configuration provides a laser which mode locks withoutrequiring a dedicated passive or active mode locking elements to beincorporated therein. By selecting the length of the resonator to becomparable with an upper-state lifetime of one or more semiconductorcarriers located within the gain medium causes a small perturbation onthe intensity of the output field of the laser which is sufficient forthe optical Kerr lensing layer to induce mode locking on the outputfield. This results in a semiconductor laser that is simpler to operateand maintain and which has reduced production costs compared with thosesystems known in the art.

The second mirror may be partially reflective and partially transmissiveat a fundamental wavelength of the gain medium so as to act as an outputcoupler for the resonator.

Most preferably the optical Kerr lensing layer comprises a heat spreadermounted upon the semiconductor gain medium. The heat spreader maycomprise a layer of diamond crystal.

The resonator may further comprise an aperture stop having an aperturelocated therein. Preferably the aperture stop is located adjacent to thesecond mirror. Alternatively the aperture is located adjacent to thefirst mirror.

The resonator may be additionally folded by a fourth mirror, the fourthmirror being located between the second and third mirrors. The fourthmirror preferably has a concave radius of curvature.

The resonator may be additionally folded by a fifth mirror, the fifthmirror being located between the second and fourth mirrors. The fifthmirror is preferably planar.

The resonator may be additionally folded by a sixth mirror, the sixthmirror being located between the second and fifth mirrors. The sixthmirror preferably has a concave radius of curvature.

Optionally the laser comprises a continuous wave (cw) optical fieldsource the output from which is configured to pump the gain medium. The(cw) optical field source may comprise a fibre coupled laser diodesystem

The resonator may further comprise an astigmatism controller thatprovides a means for introducing astigmatism to the cavity mode at thegain medium.

In this embodiment the resonator is preferably configured such that theKerr lensing layer acts to compensate for the astigmatism introduced tothe cavity mode. By compensating for the astigmatism introduced to thecavity mode the area of overlap between the cavity mode and a pump spotat the gain medium is increased. As a result the self mode lockingnature of the laser is enhanced.

The astigmatism controller may comprise a mirror rotating means. Themirror rotating means may be employed to rotate the fourth mirror so asto vary the angle of incidence of a resonating field upon the fourthmirror.

Most preferably the laser provides an output field comprising ultrashort pulses. The ultra short pulses may have a pulse width in the rangeof 100 ps to 100 fs.

According to a second aspect of the present invention there is provideda method of self mode locking a laser the method comprising

-   -   providing a resonator terminated by first and second mirrors and        folded by a third mirror, the third mirror surmounted by a        multilayer semiconductor gain medium including at least one        quantum well layer and an optical Kerr lensing layer; and    -   selecting a length of the resonator such that a round trip time        of a cavity mode corresponds with an upper-state lifetime of one        or more semiconductor carriers located within the gain medium.

The method of self mode locking a laser may further comprise locating anaperture stop having an aperture located within the resonator.

Preferably the aperture stop is located adjacent to the second mirror.Alternatively the aperture is located adjacent to the first mirror.

The method of self mode locking a laser may further comprise folding thecavity by providing a fourth mirror between the second and thirdmirrors.

The method of self mode locking a laser may further comprise folding thecavity by providing a fifth mirror between the second and fourthmirrors.

The method of self mode locking a laser may further comprise folding thecavity by providing a sixth mirror between the second and fifth mirrors.

The method of self mode locking a laser may further comprise providing acontinuous wave (cw) optical field configured to pump the gain medium.

The method of self mode locking a laser may further comprise introducingastigmatism to the cavity mode at the gain medium.

The astigmatism may be introduced to the cavity mode by rotating thefourth mirror so as to increase the angle of incidence of a resonatingfield upon the fourth mirror.

The method of self mode locking a laser may further comprise configuringthe resonator such that the Kerr lensing layer acts to compensate forthe astigmatism introduced to the cavity mode. In this way an area ofoverlap between the cavity mode and a pump spot at the gain medium isincreased.

Embodiments of the second aspect of the invention may comprise featuresto implement the preferred or optional features of the first aspect ofthe invention or vice versa.

According to a third aspect of the present invention there is provided aself mode locking laser the laser comprising

a resonator terminated by first and second mirrors and folded by a thirdmirror, the third mirror surmounted by a multilayer semiconductor gainmedium including at least one quantum well layer and an optical Kerrlensing layer,a continuous wave (cw) optical field source the output from which isconfigured to pump the gain medium,wherein a length of the resonator is selected such that a round triptime of a cavity mode corresponds with an upper-state lifetime of one ormore semiconductor carriers located within the gain medium.

Embodiments of the third aspect of the invention may comprise featuresto implement the preferred or optional features of the first or secondaspects of the invention or vice versa.

According to a fourth aspect of the present invention there is provideda method of self mode locking a laser the method comprising

-   -   providing a resonator terminated by first and second mirrors and        folded by a third mirror, the third mirror surmounted by a        multilayer semiconductor gain medium including at least one        quantum well layer and an optical Kerr lensing layer;    -   providing a continuous wave (cw) optical field configured to        pump the gain medium; and    -   selecting a length of the resonator such that a round trip time        of a cavity mode corresponds with an upper-state lifetime of one        or more semiconductor carriers located within the gain medium.

Embodiments of the fourth aspect of the invention may comprise featuresto implement the preferred or optional features of the first to thirdaspects of the invention or vice versa.

According to a fifth aspect of the present invention there is provided aself mode locking laser the laser comprising

a resonator terminated by first and second mirrors and folded by a thirdmirror, the third mirror surmounted by a multilayer semiconductor gainmedium including at least one quantum well layer and an optical Kerrlensing layer,an astigmatism controller that provides a means for introducingastigmatism to a cavity mode at the gain medium,wherein the resonator is configured such that the Kerr lensing layeracts to compensate for the astigmatism introduced to the cavity mode.

By configuring the resonator such that the Kerr lensing layer acts tocompensate for the astigmatism introduced to the cavity mode the area ofoverlap between the cavity mode and a pump spot at the gain medium isincreased. As a result the above configuration provides a laser whichmode locks without requiring a dedicated passive or active mode lockingelements to be incorporated therein. This results in a semiconductorlaser that is simpler to operate and maintain and which has reducedproduction costs compared with those systems known in the art.

The second mirror may be partially reflective and partially transmissiveat a fundamental wavelength of the gain medium so as to act as an outputcoupler for the resonator.

Most preferably the optical Kerr lensing layer comprises a heat spreadermounted upon the semiconductor gain medium. The heat spreader maycomprise a layer of diamond crystal.

The resonator may further comprise an aperture stop having an aperturelocated therein. Preferably the aperture stop is located adjacent to thesecond mirror. Alternatively the aperture is located adjacent to thefirst mirror.

The resonator may be additionally folded by a fourth mirror, the fourthmirror being located between the second and third mirrors. The fourthmirror preferably has a concave radius of curvature.

The resonator may be additionally folded by a fifth mirror, the fifthmirror being located between the second and fourth mirrors. The fifthmirror is preferably planar.

The resonator may be additionally folded by a sixth mirror, the sixthmirror being located between the second and fifth mirrors. The sixthmirror preferably has a concave radius of curvature.

Optionally the laser comprises a continuous wave (cw) optical fieldsource the output from which is configured to pump the gain medium. The(cw) optical field source may comprise a fibre coupled laser diodesystem

The astigmatism controller may comprise a mirror rotating means.

The mirror rotating means may be employed for rotating the fourth mirrorso as to vary the angle of incidence of a resonating field upon thefourth mirror.

The length of the resonator may be selected such that a round trip timeof a cavity mode corresponds with an upper-state lifetime of one or moresemiconductor carriers located within the gain medium.

Most preferably the laser provides an output field comprising ultrashort pulses. The ultra short pulses may have a pulse width in the rangeof 100 ps to 100 fs.

Embodiments of the fifth aspect of the invention may comprise featuresto implement the preferred or optional features of the first to fourthaspects of the invention or vice versa.

According to a sixth aspect of the present invention there is provided amethod of self mode locking a laser the method comprising

-   -   providing a resonator terminated by first and second mirrors and        folded by a third mirror, the third mirror surmounted by a        multilayer semiconductor gain medium including at least one        quantum well layer and an optical Kerr lensing layer;    -   introducing astigmatism to the cavity mode at the gain medium;        and    -   configuring the resonator such that the Kerr lensing layer acts        to compensate for the astigmatism introduced to the cavity mode.

The method of self mode locking a laser may further comprise locating anaperture stop having an aperture located within the resonator.

Preferably the aperture stop is located adjacent to the second mirror.Alternatively the aperture is located adjacent to the first mirror.

The method of self mode locking a laser may further comprise folding thecavity by providing a fourth mirror between the second and thirdmirrors.

The astigmatism may be introduced to the cavity mode by rotating thefourth mirror so as to increase the angle of incidence of a resonatingfield upon the fourth mirror.

The method of self mode locking a laser may further comprise folding thecavity by providing a fifth mirror between the second and fourthmirrors.

The method of self mode locking a laser may further comprise folding thecavity by providing a sixth mirror between the second and fifth mirrors.

The method of self mode locking a laser may further comprise providing acontinuous wave (cw) optical field configured to pump the gain medium.

The method of self mode locking a laser may further comprise selecting alength of the resonator such that a round trip time of a cavity modecorresponds with an upper-state lifetime of one or more semiconductorcarriers located within the gain medium.

Embodiments of the sixth aspect of the invention may comprise featuresto implement the preferred or optional features of the first to fifthaspects of the invention or vice versa.

BRIEF DESCRIPTION OF DRAWINGS

Aspects and advantages of the present invention will become apparentupon reading the following detailed description and upon reference tothe following drawings in which:

FIG. 1 presents a schematic representation of a self mode-locking,external-cavity surface-emitting, semiconductor laser in accordance withan embodiment of the present invention;

FIG. 2 presents a schematic representation of a semiconductor disk laser(SDL) employed by the laser of FIG. 1;

FIG. 3 present a schematic representation of a cooling apparatusemployed in conjunction with the SDL of FIG. 2;

FIG. 4 presents a schematic representation of a cavity mode, a pump spotand a Kerr Lens mode at the surface of the SDL of FIG. 1;

FIG. 5 presents a semiconductor laser in accordance with an alternativeembodiment of the present invention; and

FIG. 6 presents a semiconductor laser in accordance with a furtheralternative embodiment of the present invention.

In the description which follows, like parts are marked throughout thespecification and drawings with the same reference numerals. Thedrawings are not necessarily to scale and the proportions of certainparts have been exaggerated to better illustrate details and features ofembodiments of the invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, a schematic representation of a selfmode-locking, external-cavity surface-emitting, semiconductor laser 1 inaccordance with an embodiment of the present invention is shown. Forclarity of understanding axes are provided within this figure. The planeof the cavity referred to below is the plane defined by the x and zaxes.

The self mode-locking laser 1 can be seen to comprise a laser-resonator2 formed between a first 3 and a second mirror 4 and includes amultilayer, optically-pumped, semiconductor disk laser (SDL) 5 furtherdetails of which are provided below with reference to FIGS. 2 and 3. Ascan be seen the SDL 5 is arranged to function as a first folding mirrorfor the resonator 2. Three further folding mirrors 6, 7 and 8 areincluded within the resonator 2 and so the resonator 2 can be consideredto be a four times folded resonator.

The first mirror 3 and the three folding mirrors 6, 7 and 8 are arrangedto be highly reflective at the fundamental wavelength of the SDL 5 whilethe second mirror 4 is partially reflective and partially transmissiveat this fundamental wavelength and so acts as an output coupler for theresonator 2. An aperture stop 9 comprising an aperture 10, such as aslit aperture, may be located adjacent to the second mirror 4. Theaperture may be orientated in the plane of the cavity, in a planeperpendicular to the plane of the cavity or indeed comprise an iris andthus have a component in both planes.

The mirrors 3, 6, 7 and 8 may be mounted within piezo-electriccontrolled mirror mounts so as to provide a means for fine adjusting thealignment of these components. Furthermore, mirrors 3, 6 and 8 areconcave mirrors while the mirrors 4 and 7 and the SDL 5 aresubstantially planar reflecting elements such that the resonator 2 isoptically stable and the shape of the cavity mode at the SDL 5 can becontrolled, as discussed in further detail below.

A continuous wave (cw) optical pumping field 11 suitable for pumping theSDL 5 is provided by employing a fibre coupled laser diode system 12. Inthe presently described embodiment the fibre coupled laser diode system12 is configured to generate a cw optical pumping field 11 at 808 nm. ADILAS® M1F4S22-808 30C-SS2.1 is an example of one such suitable fibrecoupled laser diode system 12.

In the presently described embodiment the fibre coupled laser diodesystem 12 is arranged to pump the gain medium 18 at an angle suitablefor providing an elliptical pump spot at the surface of the gain medium18. It will be appreciated by the skilled reader that the presentinvention is not so limited and that the fibre coupled laser diodesystem 12 could provide a pump field 11 that is perpendicular to thegain medium 18 so as to provide a circular pump spot at the surface ofthe gain medium 18. The fibre coupled laser diode system 12 may also bearranged to pump the gain medium 18 by pumping through the first mirror3.

In FIG. 1 the intra cavity resonating field is depicted generally byreference numeral 13 while the ultra short pulsed output field from thelaser resonator 2 is depicted generally by reference numeral 14.

A schematic representation of the SDL 5 is presented in FIG. 2. The SDL5 can be seen to comprise a wafer structure 15 that is grown by ametal-organic chemical vapour deposition (MOCVD) technique on a GaAssubstrate 16. The deposition of the wafer structure may be achieved byalternative techniques known in the art e.g. molecular beam epitaxy(MBE) deposition techniques. The wafer structure 15 comprises a singledistributed Bragg reflector (DBR) region 17, a gain medium 18, a carrierconfinement potential barrier 19 and an oxidation prevention layer 20.

There are many variations of the wafer structures 15 incorporated withinthe SDLs known to those skilled in the art and the present invention isnot limited to use with any particular DBR region 17 or gain medium 18structure. In general, the gain medium 18 will comprise multiple quantumwells equally spaced between half-wave structures that allow the SDL 5to be optically pumped at a convenient pump wavelength while the DBRregions 17 generally comprise multiple pairs of quarter-wave layers thatexhibit high reflectivities.

The presently described embodiments comprise a gain medium 18 comprisingInGaAs quantum wells equally spaced between half-wave GaAs structuresthat allow the SDL 5 to be optically pumped at 808 nm while generatingan output at 980 nm. The DBR regions 17 comprise thirty pairs ofAlAs—GaAs quarter-wave layers that produce reflectivities greater than99.9% centred at 980 nm while the carrier confinement potential barrier19 comprises a single wavelength-thick Al_(0.3)Ga_(0.7)As layer. Theoxidation prevention layer 20 may comprise a thin GaAs cap.

Alternative gain mediums known to those skilled in the art that mayalternatively be used include alternative gallium arsenide (GaAs)structures capable of generating output wavelengths between 670 nm and1300 nm; Indium Phosphide (InP) structures capable of generating outputwavelengths between 1350 nm and 1600 nm; and Gallium Antimonide (GaSb)structures capable of generating output wavelengths between 1800 nm and2700 nm. These gain mediums may be based on quantum wells or quantumdots as known to those skilled in the art.

For reasons as will be described in further detail below, the length ofthe resonator 2 may be selected such that the round trip time of thecavity mode corresponds to the upper-state lifetime of the semiconductorcarriers located within the gain medium 18. In the presently describedembodiment the lifetime of the semiconductor carries is around 5 ns,giving the resonator a length of around 750 mm and a repetition rate ofaround 200 MHz.

This arrangement is counter intuitive to the teachings within the artwhere it is generally desirable to make the length of a resonator assmall as possible so as assist in the overall miniaturisation of thesystem. The main restrictions on the minimum length of a resonator isthe requirement to provide sufficient physical space for all of theoptical components required to be incorporated into the system and toallow for the desired cavity mode characteristics to be achieved. Insystems known in the art the selected resonator lengths result in around trip time for the cavity mode that is much lower than theupper-state lifetime of the associated gain medium, normally by severalorders of magnitude.

FIG. 3 presents further detail of a cooling apparatus 21 employed inorder to improve the operating characteristics of the SDL 5. Inparticular, the cooling apparatus 21 comprises a heat spreader 22 and astandard thermoelectric or water cooler 23. The heat spreader 22comprises a single diamond crystal that comprises an external, wedgedface 24.

A high performance anti-reflection coating may be deposited on thesurface of the wedged face 24.

The single diamond crystal heat spreader 22 is bonded by opticalcontacting with the wafer structure 15 so that the gain medium 18 islocated between the heat spreader 22 and the DBR region 17. The waferstructure 15 and heat spreader 22 are then fixed on top of a layer ofindium foil 25 onto the thermoelectric or water cooler 23.

Single diamond crystal is well suited to be employed as the heatspreader 22 since it exhibits comparable thermal conductivity levels assapphire and silicon carbide. Thus, the described arrangement allows theheat spreader 22 to immediately spread the heat generated within thegain medium 18 by the pump field 11 to the cooling apparatus 21 after ithas propagated only a limited distance into the gain medium 18. As aresult the overall efficiency of the SDL 5 is significantly increased.

In addition there is a further inherent advantage of employing thesingle diamond crystal as the heat spreader 22. This resides in the factthat the single diamond crystal is a material that exhibits an inherentoptical Kerr effect. It is this effect that is exploited in order toconfigure the semiconductor laser 1 so as to operate as a self modelocking system, as will now be described in further detail withreference to FIG. 4.

In particular, FIG. 4 presents a schematic representation of a cavitymode 26, a pump spot 27 and a Kerr Lens mode 28 at the surface of theSDL 5 of FIG. 1. The laser is configured such there is an overlap of thearea of the cavity mode 26, the pump spot 27 and the Kerr Lens mode 28at the surface of the SDL 5.

The area of the Kerr lens mode 28 at the SDL 5 is defined by the singlediamond crystal heat spreader 22 and in the presently describedembodiment it exhibits an elliptical profile with its major axisorientated along the x-axis. In a similar manner the pump spot 27 at theSDL 5 is configured to have an elliptical profile with its major axisalso orientated along the x-axis. The major axis of the Kerr lens mode28 in the presently described embodiment is smaller than the major axisof the pump spot 27.

The concave folding mirror 6 is arranged so as to introduce astigmatismto the cavity mode 26. This is achieved by rotating the concave foldingmirror 6 about the y-axis so as to increase the angle of incidence ofthe resonating field 13 upon this mirror 6. As can be seen from FIG. 4,this rotation results in the cavity mode 26 at the SDL 5 having anelliptical profile with its major axis orientated along the y-axis.

In this configuration the semiconductor laser 1 begins to lase when thegain medium 18 is pumped by the pumping field 11 and the output field 14is thus generated. Most significant is that the laser is self modelocking such an ultra short output field at 980 nm is produced i.e.pulse widths from 100 ps down to a few femtoseconds can be generated.This result is highly repeatable and the mode locking takes placewithout any requirement for further input from the operator of the laser1.

The inventors believe that there exist two independent mechanisms whichcontribute to allow for self mode locking of the laser 1. In thepresently described laser 1 these mechanisms are acting in combinationbut they may alternatively be independently exploited.

The first mechanism for the self mode locking of the laser 1 resultsfrom the fact that length of the resonator 2 is selected such that theround trip time of the cavity mode 26 is close to the upper-statelifetime of the semiconductor carriers located within the gain medium18. This introduces a small perturbation on the intensity of the outputfield 14 which is sufficient for the small inherent optical Kerr effectof the single diamond crystal heat spreader 22 to induce mode locking onthe output field 14.

This process is further assisted by the second mechanism which residesin the introduction of the astigmatism to the cavity mode 26 at thesurface of the SDL 5. Once the Kerr lensing effect of the heat spreader22 commences the major axis of the cavity mode 26 is effectively reducedthus causing a greater overlap between the area of the cavity mode 26and the pump spot 27. Thus, by employing the Kerr lensing effect of theheat spreader 22 to overcome an induced astigmatism a second means forself mode locking of the output field 14 is provided.

As will be appreciated by the skilled reader both of these mechanismsmay be assisted by the presence of the aperture stop 9 when the aperture10 is configured such that the lasing mode of the resonator at theaperture 10 is clipped and lasing is not possible in the absence of theKerr effect induced by the heat spreader 22. It will be furtherappreciated that the aperture stop 9 could alternatively be locatedadjacent to the first mirror 3.

FIG. 5 presents a schematic representation of a self mode-locking,external-cavity surface-emitting, semiconductor laser 29 in accordancewith an alternative embodiment of the present invention, similar to thelaser 1 presented in FIG. 1. In this embodiment folding mirrors 8 hasbeen replaced by the output coupler 4 such that the resonator 2 b cannow be considered to be a three times folded resonator.

FIG. 6 presents a schematic representation of a self mode-locking,external-cavity surface-emitting, semiconductor laser 30 in accordancewith a further alternative embodiment of the present invention, similarto the laser 1 presented in FIG. 1. In this embodiment folding mirrors 8has been omitted and folding mirror 7 has been replaced by the outputcoupler 4 such that the resonator 2 c can now be considered to be a twotimes folded resonator.

It will be appreciated that a number of alternatives may be incorporatedinto the above described embodiments. For example the structure of theSDL 5 may be varied so as to provided alternative output wavelengths asrequired by the particular application for which the semiconductor laseris to be employed.

Furthermore, the orientations of the cavity mode 26, the pump spot 27and the Kerr Lens mode 28 may be varied such that that the anglesbetween the associated major axes vary from the particular describedembodiment. What is important is that the resonator is configured suchthat an astigmatism introduced to the cavity mode 26 by theconfiguration of the resonator 2 is reduced by the optical Kerr effectinduced by the heat spreader 22 when the gain medium 18 is pumped by thepumping field 11 such that the overlap area between the cavity mode 26and the pump spot 27 is increased.

The heat spreader may alternatively comprise materials other than singlediamond crystal as long as the material employed exhibits the requiredheat spreading and optical Kerr lensing properties. Sapphire (Al₂O₂) andsilicon carbide (SiC) are examples of alternative materials that may beemployed to produce the heat spreader.

The described semiconductor lasers offer a number of advantages overthose known in the art. When compared to the previously described solidstate ultrafast lasers the presently described systems are significantlyless complex, more compact, have reduced maintenance requirements andare significantly less expensive to produce.

The fact that the described semiconductor lasers are self mode lockingalso removes the requirement for dedicated passive or active modelocking elements to be incorporated. This again results in the presentlydescribed semiconductor lasers having a reduced complexity, maintenancerequirement and associated production costs.

The presently described semiconductor laser systems can be employed togenerate pulses having a pulse widths ranging from 100 ps to ˜100 fs, atwavelengths between 670 nm and 2700 nm and with power outputs rangingfrom 100 mW to 5 W.

The above factors make the described semiconductor lasers ideal for usewithin nonlinear microscopy techniques e.g. Two-Photon ExcitedFluorescence (TPEF) microscopy or other similar multi-photon microscopytechniques. For example the short pulse widths allow for significantdepth profiling to be performed on Green Fluorescent Proteins (GFPs)which exhibit excitation peaks at 395 nm and 475 nm or 496 nm dependingon the particular GFP employed.

The present invention describes a self mode locking laser and a methodfor self mode locking a laser. The laser comprises a resonatorterminated by first and second mirrors and folded by a third mirror. Thethird mirror comprises a single distributed Bragg reflector (DBR) uponwhich is mounted a multilayer semiconductor gain medium and whichincludes at least one quantum well layer and an optical Kerr lensinglayer. Self mode locking may be achieved by selecting the length of theresonator such that a round trip time of a cavity mode is matched withan upper-state lifetime of one or more semiconductor carriers locatedwithin the gain medium. The self mode locking of the laser may befurther enhanced by configuring the laser resonator such that thelensing effect of the Kerr lensing layer acts to reduce an astigmatismdeliberately introduced to the cavity mode.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed. Thedescribed embodiments were chosen and described in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilise the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. Therefore, further modifications orimprovements may be incorporated without departing from the scope of theinvention as defined by the appended claims.

1. A self mode locking laser the laser comprising a resonator terminatedby first and second mirrors and folded by a third mirror, the thirdmirror surmounted by a multilayer semiconductor gain medium including atleast one quantum well layer and an optical Kerr lensing layer, whereina length of the resonator is selected such that a round trip time of acavity mode corresponds with an upper-state lifetime of one or moresemiconductor carriers located within the gain medium.
 2. A self modelocking laser as claimed in claim 1 wherein the second mirror ispartially reflective and partially transmissive at a fundamentalwavelength of the gain medium so as to act as an output coupler for theresonator.
 3. A self mode locking laser as claimed in claim 1 whereinthe optical Kerr lensing layer comprises a heat spreader mounted uponthe semiconductor gain medium.
 4. A self mode locking laser as claimedin claim 3 wherein the heat spreader comprises a layer of diamondcrystal.
 5. A self mode locking laser as claimed in claim 1 wherein theresonator further comprises an aperture stop having an aperture locatedtherein.
 6. A self mode locking laser as claimed in claim 5 wherein theaperture stop is located adjacent to the second mirror.
 7. A self modelocking laser as claimed in claim 5 wherein the aperture is locatedadjacent to the first mirror.
 8. A self mode locking laser as claimed inclaim 1 wherein the resonator is additionally folded by a fourth mirror,the fourth mirror being located between the second and third mirrors. 9.A self mode locking laser as claimed in claim 8 wherein the fourthmirror has a concave radius of curvature.
 10. A self mode locking laseras claimed in claim 8 wherein the resonator is additionally folded by afifth mirror, the fifth mirror being located between the second andfourth mirrors.
 11. A self mode locking laser as claimed in claim 10wherein the fifth mirror is planar.
 12. A self mode locking laser asclaimed in claim 10 wherein the resonator is additionally folded by asixth mirror, the sixth mirror being located between the second andfifth mirrors.
 13. A self mode locking laser as claimed in claim 12wherein the sixth mirror has a concave radius of curvature.
 14. A selfmode locking laser as claimed in claim 1 wherein the laser furthercomprises a continuous wave (cw) optical field source the output fromwhich is configured to pump the gain medium.
 15. A self mode lockinglaser as claimed in claim 14 wherein the (cw) optical field sourcecomprises a fibre coupled laser diode system.
 16. A self mode lockinglaser as claimed in claim 1 wherein the resonator further comprises anastigmatism controller that provides a means for introducing astigmatismto the cavity mode at the gain medium.
 17. A self mode locking laser asclaimed in claim 16 wherein the resonator is configured such that theKerr lensing layer acts to compensate for the astigmatism introduced tothe cavity mode.
 18. A self mode locking laser as claimed in claim 16wherein the astigmatism controller comprises a mirror rotating means.19. A self mode locking laser as claimed in claim 18 wherein the mirrorrotating means is employed to rotate the fourth mirror so as to vary anangle of incidence of a resonating field upon the fourth mirror.
 20. Aself mode locking laser as claimed in claim 1 wherein the laser providesan output field comprising ultra short pulses having a pulse width inthe range of 100 ps to 100 fs.
 21. A method of self mode locking a laserthe method comprising providing a resonator terminated by first andsecond mirrors and folded by a third mirror, the third mirror surmountedby a multilayer semiconductor gain medium including at least one quantumwell layer and an optical Kerr lensing layer; and selecting a length ofthe resonator such that a round trip time of a cavity mode correspondswith an upper-state lifetime of one or more semiconductor carrierslocated within the gain medium.
 22. A method of self mode locking alaser as claimed in claim 21 wherein the method further compriseslocating an aperture stop having an aperture located therein within theresonator.
 23. A method of self mode locking a laser as claimed in claim22 wherein the aperture stop is located adjacent to the second mirror.24. A method of self mode locking a laser as claimed in claim 22 whereinthe aperture is located adjacent to the first mirror.
 25. A method ofself mode locking a laser as claimed in claim 21 wherein the methodfurther comprises folding the cavity by providing a fourth mirrorbetween the second and third mirrors.
 26. A method of self mode lockinga laser as claimed in claim 25 wherein the method further comprisesfolding the cavity by providing a fifth mirror between the second andfourth mirrors.
 27. A method of self mode locking a laser as claimed inclaim 26 wherein the method further comprises folding the cavity byproviding a sixth mirror between the second and fifth mirrors.
 28. Amethod of self mode locking a laser as claimed in claim 21 wherein themethod further comprises providing a continuous wave (cw) optical fieldconfigured to pump the gain medium.
 29. A method of self mode locking alaser as claimed in claim 21 wherein the method further comprisesintroducing astigmatism to the cavity mode at the gain medium.
 30. Amethod of self mode locking a laser as claimed in claim 29 wherein theastigmatism is introduced to the cavity mode by rotating the fourthmirror so as to increase the angle of incidence of a resonating fieldupon the fourth mirror.
 31. A method of self mode locking a laser asclaimed in claim 29 wherein the method further comprises configuring theresonator such that the Kerr lensing layer acts to compensate for theastigmatism introduced to the cavity mode.